Device for detecting an electromagnetic radiation with current limitation

ABSTRACT

This device for detecting an electromagnetic radiation, comprises a matrix of juxtaposed elementary sensors ( 1 ), each associated with a common substrate in which a sequential addressing read circuit is prepared, specific to each of the sensors, thereby constituting as many pixels, the interaction of the radiation with the sensors generating electric charges to be converted to voltage for their subsequent processing, each of the said sensors being biased via an injection transistor ( 2 ), of which one of the terminals is connected to an integration capacitance ( 3 ), storing the electric charges generated by the sensor during an integration phase, and whereof the quantity of charges is then processed for conversion to voltage. Each of the pixels of the said matrix is associated with a current limiting device ( 5 ), for limiting the current generated by each of the elementary sensors to a maximum called reference current, regardless of the radiation flux received by the pixel concerned.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U. S. C. §119 from FrenchPatent Application No. 0757589 filed on Sep. 14, 2007 in the FrenchPatent Office, the entire disclosure of which is incorporated herein byreference.

FIELD OF THE INVENTION

The invention relates to a device for detecting an electromagneticradiation, more particularly for detecting infrared radiation,ultraviolet radiation, X-rays, and in the visible region.

This device uses sensors sensitive to such radiations, and for theinfrared region, uses quantum detectors, that are operating at very lowtemperature (close to liquid nitrogen) as opposed to ambient temperatureinfrared detectors, which use bolometers.

BACKGROUND OF THE INVENTION

In the field of electromagnetic radiation detection, it is well knownhow to use devices arranged in matrix form, that is comprising aplurality of juxtaposed elementary sensors in order to form a matrixhaving a number of lines and columns. The interactions of theelectromagnetic radiations with these elementary sensors, also called“pixels,” generate flows of charge carriers, of which the energy and/orquantity corresponds to the energy of the incident radiation. Typically,the charge carriers consist of electrons or conduction gaps.

The charge carrier flux flows towards the read circuit associated withthe sensor to form analog signals. These signals are then processed forsubsequent reconstruction of a visible image, a function of the incidentlighting.

An imager is thus produced, consisting of a matrix of pixels sensitiveto a predefined range of electromagnetic radiations.

FIG. 1 shows the read circuit associated with an elementary detector inthe infrared region, the said detector typically consisting of aphotodiode, and using a MOSFET injection transistor.

In the present case, the photodiode dph 1 is subjected to a bias voltagevia a voltage applied to the grid G of an injection transistor Minj 2,in the present case consisting of an N type MOSFET, connected to thephotodiode dph 1 via its source S. The drain D of the injectiontransistor 2 is connected to an integration capacitance Cint 3.

When the photodiode 1 is subjected to a radiation with an energy hυ andfluxΦ, the said radiation causes the photodiode to generate a currenti(Φ)) which passes through the injection transistor 2 by matching isvoltage VGS, and also the integration capacitance Cint 3.

During the integration time T, the current i(Φ) charges the capacitanceCint with charge carriers according to the equation

Qint(Φ, T) = ∫₀^(T)i(Φ), t.

A switch 4 serves to transfer the charge Qint((Φ,T) from the capacitance3, for example to an amplifier for converting this charge to voltage, ina manner known per se.

It is obviously possible to use a P type MOSFET injection transistor, oreven junction gate field effect transistors (JFET), and even bipolartransistors. These various transistors may be N or P doped, and may evenbe enriched or depleted.

If the incident photon flux is too intense, the current generated in theelementary detector rapidly becomes too high, and a depolarization ofall the detectors of the matrix is observed. In such a situation, thematrix is then bloomed and the reconstruction of the visible image iscompromised, or even impossible. In other words, the whole image becomesblack even though a single, or even a few of the component pixels areaffected by this blooming.

In order to overcome this malfunction, anti-blooming devices have beenproposed, based on a clamping of the surplus current, but which does notprevent the saturation of the detector itself.

It is the object of the present invention to propose a detection deviceof the type in question, which overcomes these blooming problems, butwithout acting on the actual detector, but more on the electroniccircuitry associated with the said detector, and therefore moreparticularly on the read circuit.

SUMMARY OF THE INVENTION

The invention relates to a device for detecting an electromagneticradiation, comprising a matrix of juxtaposed elementary sensors, eachassociated with a common substrate in which a sequential addressing readcircuit is prepared, specific to each of the sensors, the interaction ofthe radiation with the sensors generating electric charges to beconverted to voltage for their subsequent processing, each of the saidsensors being biased via an injection transistor, of which one of theterminals is connected to an integration capacitance, storing theelectric charges generated by the sensor during an integration phase,and whereof the quantity of charges is then processed for conversion tovoltage.

According to one feature of the invention, each of the pixels of thesaid matrix, consisting of an elementary sensor and its specific readcircuit, is associated with a current limiting device, for limiting thecurrent generated by each of the elementary sensors to a maximum calledreference current, regardless of the radiation flux received by thepixel concerned.

In other words, the invention consists not in acting on the elementarysensors, but directly on the associated read circuit, in such a way asto cause, in the sensor concerned, a variation in the bias voltage,suitable, if necessary, for decreasing the sensitivity of the detectorto an excessively high flux of detected radiation.

According to the invention, the current limiting device receives twoinput signals, the measurement of the detector current and the referencecurrent respectively, and in that it transmits an output signal,consisting of a physical quantity suitable for controlling the injectiontransistor.

According to the invention, this current limiting device comprises atleast: a current comparator, for comparing the current of the sensorcharging the integration capacitance with the reference value, and acircuit for modifying the grid voltage of the injection transistoraccording to the comparison thus made.

According to the invention, the current limiting device is mounted infeedback with regard to the bias circuit of the elementary sensor. Dueto its feedback arrangement, the system balances itself alone withoutexternal action. In fact, due to this feedback, a servocontrolled andstable system is obtained: an increase in the output signal causes adecrease in the input stimulus. The reference current is matchable by acurrent source i_(ref), acting on the comparator. The value of thiscurrent is selected according to the value of the flux to be detected,the sensitivity, and the type of detector.

Advantageously, the sensor current is duplicated by means of a currentmirror, connected to the input of the current comparator, the otherinput of the said comparator receiving the reference current i_(ref),the said comparator making a summation between the duplicated currentand the reference current, of which the resulting current is in turnamplified and converted to bias voltage via the injection transistor.

BRIEF DESCRIPTION OF THE FIGURES

The manner in which the invention can be implemented and the advantagesthereof will appear more clearly from the exemplary embodiment thatfollows, given for information and non-limiting, in conjunction with theappended figures.

FIG. 1, as already stated, is a schematic representation of the readcircuit of an elementary sensor of a detector according to the priorart.

FIG. 2 is a schematic representation illustrating the operatingprinciple of the current limiter according to the invention.

FIG. 3 is a schematic representation illustrating one possibleembodiment of the current limiter of the invention, implementing theCMOS microelectronic technology.

FIG. 4 shows a set of graphs illustrating the variation in current andvoltage at the terminals of some of the components of an elementarysensor or of its associated read circuit, highlighting the action of thecurrent limiter according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2 illustrates the general principle of the detection deviceaccording to the invention.

The rest of the description is more concerned with describing aninfrared detector using photodiodes. However, it is obvious that aperson skilled in the art is capable of modifying the componentsrequired to adapt the said components to the type of radiation to bedetected, in particular X-rays, visible region, ultraviolet region.

In the present case, a photodiode 1 is used of the same type and samecharacteristics as the one described in conjunction with FIG. 1.

As may be observed, the current limiter 5 according to the invention ismounted in feedback on the grid G of the injector transistor 2. For thispurpose, it first comprises a current mirror 6 consisting (cf. FIG. 4)of a transistor M1 and a transistor M2 for which current ratios equal to1 are selected, but in a non-limiting manner.

The current mirrors are well known to a person skilled in the art, sothat there is no need to describe them in greater detail here. It shouldsimply be recalled that such a current mirror is a particular circuit,in the present case designed with two transistors, for reflecting(duplicating) the initial current, in the present case issuing from theintegration capacitance 3, into an output current, in the present caseequal, or in any case proportional to the incident current,independently of the load connected to the output current, and in thepresent case to the comparator 7 described below in greater detail.

In other words, the current in the transistor M2 is the image of thecurrent i(Cint), so that:

ids(M2)=ids(M1)=i(Cint).

According to the invention, a reference current i_(ref) is injected bymeans of a current source (not shown) and to do this, a current mirroris also used, consisting of two transistors M3 and M4 as may be observedin FIG. 3. Thus, the current in the transistor M4 is the image of thetransistor M3 with which it forms the current mirror.

Obviously, for a simpler device, such a current mirror is notindispensable. Thus, the reference current may be applied directly tothe drain of the transistor M5, described below.

Advantageously, the transistor M3 may be common to several pixels andtherefore placed outside the pixel concerned, as a shared resource.

The current comparator 7 consists in the present case of the transistorM5 (FIG. 3) which summates the input current ids(M2) corresponding, asalready stated, to the current issuing from the integration capacitance3, and the output current ids(M4) corresponding to the referencecurrent, giving the equation:

ids(M5)=ids(M2)−ids(M4)=ids(Cint)−iref

In doing so, if the current issuing from the transistor M2 is higherthan the current issuing from the transistor M4, that is if the currentissuing from the integration capacitance 3 is higher than the currentsetpoint or reference current i_(ref), then the current in M5 ispositive. In the opposite case, the current in M5 is zero, the referencecurrent i_(ref) thus constituting a threshold.

A new current mirror, consisting in the present case of transistors M5and M6, duplicates the current resulting from the transistor M5.

An amplifier consisting of the transistor M7, of which the source isconnected to the bias voltage, is traversed by the current issuing fromthe current mirror M6 M5 and generates a voltage applied to the grid Gof the injection transistor 2 according to the equation:

V(G)=V(pol)−(1/GM7)·ids(M7)=V(pol)−(R7)·ids(M7).

Finally, V(G)=V(pol)−(R7)·(ids(Cint)−iref)).

In doing so, if the current issuing from the integration capacitance 3exceeds the value of the threshold current iref, then it is limited tothis value. If, on the other hand, this current is lower than thethreshold current, no modification of the bias voltage occurs at thegrid G of the injection transistor 2.

It may be observed that when the integration capacitance is filled orsaturated, the drain potential of the injection transistor 2 descends toVds=0. The transistor is then in blocked mode and prevents the currentfrom passing through: the integration capacitance 3 no longer acceptsany charge.

In a particular exemplary embodiment, the dimensions of the variousMOSFET transistors of the current limiting device for a 0.5 μmtechnology may be expressed for example in μm: Minj−3/1; M1 to M6: 1/1;M7:1/5

-   -   W/L=3/1 for Minj    -   W/L=1/1 for M4 to M6    -   W/L=1/5 for M7

The above assembly produces a feedback effect on the grid G of theinjection transistor 2. In doing so, two states are accordinglypossible: below the value of the reference current, the feedback isinactive: the read circuit then respects the integrity of the currentproduced by the photon flux; above the reference current value, acurrent variation in the integration capacitance causes a potentialvariation on the grid of the injection transistor 2, this variation inturn causing a similar variation in the bias voltage of the detector,which tends to decrease the detector current and the permissible currentto the value of the reference current.

FIG. 4 shows the translation of the action of the current limiteraccording to the invention in the form of curves.

Thus the first curve shows the variation in current resulting from thephoton flux at the input of the circuit, that is the current resultingfrom the charge carriers generated by the photodiode.

More precisely, such a variation is shown with a reference photodiode,not connected to the read circuit, and the same variation with aphotodiode associated with the read circuit according to the invention.

It may thus be observed that for a threshold current fixed at 200 nA,the respective curves are superimposed between the reference current ofthe photon flux and the current passing through the photodiode. Abovethis current, on the other hand, there is a limitation of the currentgenerated by the photodiode to this value (broken lines).

The next curve shows the variation in the grid voltage of the transistorM6. It may be observed that when the reference current is reached, thisvoltage suddenly increases because the duplicated current has justexceeded the reference current.

The third curve shows the variation in the grid voltage of the injectiontransistor. It may thus be observed that when the threshold current isexceeded, the bias voltage is decreased in order to reduce the bias ofthe photodiode concerned.

The final curve shows the variation in voltage at the terminals of theintegration capacitance 3. It may be observed that the voltage ismeasured even when the threshold is reached. There is no interruption ofthe operation of the pixel.

The invention has various advantages.

Firstly, it may be observed that with this device, the problem ofdepolarization of detectors is overcome by using an electronic solution,in the absence of any direct action on the elementary sensors. Thisovercomes the problems of blooming, such as for example the attack of aninfrared detector by a laser signal, which is particularly focused bydefinition.

Also important is the fact that even with the presence of the device ofthe invention, the measurement of the detector below the threshold isnot degraded.

Furthermore, the only data lost relates to the pixel concerned havingundergone the blooming, and not the entire matrix.

1. A device for detecting an electromagnetic radiation, comprising amatrix of juxtaposed elementary sensors, each associated with a commonsubstrate in which a sequential addressing read circuit is prepared,specific to each of the sensors, thereby constituting as many pixels,the interaction of the radiation with the sensors generating electriccharges to be converted to voltage for their subsequent processing, eachof the said sensors being biased via an injection transistor, of whichone of the terminals is connected to an integration capacitance, storingthe electric charges generated by the sensor during an integrationphase, and whereof the quantity of charges is then processed forconversion to voltage, wherein each of the pixels of the said matrix isassociated with a current limiting device, for limiting the currentgenerated by each of the elementary sensors to a maximum calledreference current, regardless of the radiation flux received by thepixel concerned.
 2. The device for detecting an electromagneticradiation according to claim 1, wherein the current limiting devicereceives two input signals, the measurement of the detector current andthe reference current respectively, and in that it transmits an outputsignal, consisting of a physical quantity suitable for controlling theinjection transistor.
 3. The device for detecting an electromagneticradiation according to claim 1, wherein the current limiting device ismounted in feedback with regard to the bias circuit of the elementarysensor.
 4. The device for detecting an electromagnetic radiationaccording to claim 1, wherein the current limiting device comprises atleast: a current comparator, for comparing the current of the sensorcharging the integration capacitance with the reference value; and acircuit for modifying the grid voltage of the injection transistoraccording to the comparison thus made.
 5. The device for detecting anelectromagnetic radiation according to claim 4, wherein the sensorcurrent is duplicated by means of a current mirror, connected to theinput of the current comparator, the other input of the said comparatorreceiving the reference current i_(ref), the said comparator making asummation between the duplicated current and the reference current, ofwhich the result is in turn amplified and converted to bias voltage viathe injection transistor.